Big Think Interview With Robert Kirshner

Robert P. Kirshner is Harvard College Professor of Astronomy and Clowes Professor of Science at Harvard University. He graduated from Harvard College in 1970 and received a Ph.D. in Astronomy at Caltech. He was a postdoc at the Kitt Peak National Observatory, and was on the faculty at the University of Michigan for 9 years. In 1986, he moved to the Harvard Astronomy Department. He served as Chairman of the Department from 1990-1997 and as the head of the Optical and Infrared Division of the CfA from 1997-2003.

Professor Kirshner is an author of over 200 research papers dealing with supernovae and observational cosmology. His work with the "High-Z Supernova Team" on the acceleration of the universe was dubbed the "Science Breakthrough of the Year for 1998" by Science Magazine. Kirshner and the High-Z Team shared in the Gruber Prize for Cosmology in 2007. A member of the American Academy of Arts and Sciences, he was elected to the National Academy of Sciences in 1998 and the American Philosophical Society in 2004. He served as President of the American Astronomical Society from 2003-2005. Kirshner's popular-level book "The Extravagant Universe: Exploding Stars, Dark Energy, and the Accelerating Cosmos" won the AAP Award for Best Professional/Scholarly Book in Physics and Astronomy and was a Finalist for the 2003 Aventis Prize.

17 March, 2010

Robert Kirshner: Well, yes. I’m Robert Kirshner and I have a lot of titles actually. I don’t know which one you want. I’m Clowes Professor of Science. How about that? Science. Here at Harvard University. I’m really an astronomy professor.

Question: Can you provide a brief tour of the major objects visible in the universe?

Robert Kirshner: Sure. If you look out at the nighttime sky, what you see right away are a few bright things that are planets in our own solar system or possibly the Moon, which is a terrible thing. We hate to have the Moon because all that reflected light makes the sky bright; makes it hard to do astronomy for distant objects. So, the real black belt astronomers don’t like the Moon; don’t like the planets very much. But, if you go out at night, you’ll see stars. And the stars that you see emitted their light, tens or hundreds, or even thousands of years ago. The speed of light, which everybody thinks is so fast, is really extremely slow. And it’s what lets astronomers see back into the past.

So, for example, the speed of light is a foot, that’s a unit of distance, used here in the United States and I believe also in Myanmar. It’s a foot in a nanosecond, or a billionth of a second. So, you never see things the way they are, you always see the past. You always see light that bounced off somebody 10 nanoseconds ago, or in the back of a big room a hundred nanoseconds ago. When you go outside, you’re seeing light that was emitted – the sunlight that was emitted eight minutes ago, light in the solar system maybe up to an hour ago. And when you look at the stars, even the nearby stars, even the bright stars, the light has been traveling to your for tens of years, or hundreds of years, or even thousands of years. So, without a telescope you can see in the past a few thousand years. And what happened in the 1920’s was that people began to realize that the system of stars that we are in, which is the Milky Way; the Milky Way Galaxy we call it today, which we see as a band of light in the summer sky because we’re looking at this system, which is a big flattened system; kind of like a pizza edge on, except we’re a pepperoni. We’re on the pizza. And so our view of it is really quite awkward. We don’t have a good perspective of the Milky Way Galaxy. But we know now that it’s roughly speaking 100,000 light years in dimension across our Milky Way. So that means it takes light 100,000 years to travel across that span of distance. And that’s really just the beginning.

What people discovered in the 1920’s was that our galaxy is just one of billions of galaxies out there. The distances between the galaxies are a few times their own diameter. So, if the galaxy is this big, then the distance to the next galaxy is kind of ten times the size of the galaxy. So, if this is 100,000 light years, the distance to the next galaxy is a few million light years. And that’s fairly accurate. The galaxy that you can see – there’s one galaxy you can see without a telescope, if you know where to look, with binoculars. And easy object in a small telescope, and that’s the Andromeda Galaxy, M31. And in the autumn sky you can pick it out. It’s kind of a fuzzy patch. What we know is that is as big a system as the one we live in. It’s as big as the Milky way; it looks like a little tiny fuzzy patch because it’s so far away.

And that’s really just the beginning. That’s our local neighborhood a few million light years away. It turns out that with modern telescopes and the best instruments and the better detectors we have today, it’s not that hard to see things that are a few billion light years away, or to measure the light from them anyway. So, that’s a thousand times farther away, it means the objects appear a million times dimmer. But what has changed over time is that we have big telescopes that collect a lot of light, and we have detectors that are nearly perfect at measuring the light and turning it into an electronic signal. So, very similar to the detectors that are in digital cameras and so on; they are made of silicon they work pretty much the same way, but we take long time exposures and we add up the data very carefully.

Anyway, we’re able to make this – the technology has enabled us to make this leap so that we can study the distant objects. And the reason why we want to do that is that the telescope is really a king of no nonsense time machine. It let’s you see the way things were in the past. Of course, it doesn’t let you see into the future. It only lets you see the past, but we can do that to distances of a billion light years, and even with some effort, to many billions of light years. And that’s important because the time since the beginning of cosmic expansion, since the beginning of the universe as we know it, the time of the Big Bang, we think is about 14 billion years ago. So the biggest distance that light could travel in that time is about 14 billion light years. And we can see things most of the way back. That means we’re not just guessing that the universe has changed over time. The universe has expanded over time; has it gotten elaborated over time due to the action of gravity pulling stuff together and stars making more complicated elements and all that stuff that’s happened over the past 14 billion years is not just a story, it’s a real history that we can observe.

Question: What are supernovae and why does your research focus on them?

Robert Kirshner: Yeah. So, stars have a lifetime. It’s very long compared to ours, so stars seem permanent. For poets, stars are kind of symbols of permanence. The speed of light is a symbol of something going fast. But for astronomers, the speed of light is slow, that’s what lets us see the past and the stars are not permanent; they are changing over time. It’s just the time scales are much longer than human lifetimes. So, a star, like the Sun, gets its energy from nuclear fusion. Down in the center of the Sun, hydrogen, which is what the sun is made of, is being converted into helium, the next element up. When you do that – when the sun does that, it generates energy. Energy is released because the helium in the nucleus has a little less mass in it then the ingredients, the hydrogen nuclei that went into it. So, hydrogen goes in and helium comes out, and the difference shows up as energy.

You know that equation that Einstein has, E=MC2, that’s not just a symbol of scientific inscrutability. That is actually an equation where the thing on the left is equal to the thing on the right, the amount of energy that you get out is equal to the change in mass times the speed of light, squared. Well, the speed of light is a pretty big number and when you square it, it’s big, big. And that means you get a lot of energy out from a small change in the mass. This is really important. So this is nuclear fusion. This is how the sun works and since the sun is made of that fuel, it can last for a very long time.

We think the lifetime for a star like the sun is about 10 billion years. So, the sun formed about 5 billion years ago, along with the planets like we’re on, and it has about 5 billion years to go. So check you’re homeowner’s policy, but I think we’re going to be okay for awhile. The interesting thing is that the more massive stars have shorter lifetimes. So, a star that has ten times the mass that the sun has a lifetime that is not measured in billions of years but only millions of years. And the very massive stars have very short lifetimes compared to the age of the earth, or compared to the age of the sun. so that means that there’s a complicated story. When stars form, the massive stars live fast, dies young; blow up, it turns out at the end. They explode as supernova explosions.

There’s another path that also leads to explosion where a star uses its fuel and crunches down in the center to become a white dwarf, which is a very dense kind of star that is left over after an ordinary star has used its nuclear fuel. The sun will probably be a white dwarf five billion years from now. In between, it will become a red giant. It will swell up and it will vaporize all of the planets, so if you’re worried about this sort of thing. You should worry. It will heat up the earth; it will boil off the oceans, melt the mountains, all that apocalyptic stuff will happen. But anyway, the lifetimes of stars are finite. The massive stars live short times, and the thing that is left over from a star like the sun is this white dwarf, which it turns out, is actually a thing that is held up by quantum mechanical forces. It’s stable if it’s left to its own devices and the white dwarf that is left after the sun has burned out, this kind of ash, this clinker that’s left over, will last indefinitely into the future. Except, some of these white dwarfs – a lot of stars are in binaries. So, the sun is a single star, it’s all by itself and has a few planets around it, but there are other stars that when they are born are born in multiple systems. In a litter of other stars. And it’s interesting because the other star can affect what happens. For example. If there’s a white dwarf in a binary system with another star, some mass from that other star, when it swells up to become a red giant can fall onto the white dwarf. And it turns out, that can make it explode as a thermonuclear bomb. So, there are stars which, left to their own devices will be fine, but in these binary systems they will explode.

So there’s a particular kind of explosion, these thermonuclear supernovae and these exploding white dwarfs, which have turned out to be incredibly important for measuring the size of the universe and for learning what the history of cosmic expansion has been.

It comes to something like this, that we judge the distances to objects from their brightness. You can go outside at the seashore and you look out and you see that there are ships out there. And you judge – you can’t really see much detail at night, but you can judge which are near and which are far with the apparent brightness of their lights. And in something like that, you can use the brightness of objects to judge their distance. The trouble with that is, it only works well if the objects really are the same brightness. Otherwise, if you have something that is intrinsically very bright, you’ll think, oh it must be nearby. And if something is intrinsically faint, you’ll say, oh that must be very far away.

So if you have objects of different brightness, it’s much harder to sort them out. And it turns out that these exploding stars, these thermonuclear supernovae, the ones where a white dwarf blows up suddenly and for a little while shines as brightly as about four billion stars like the sun, these things turn out the have a fairly narrow range in brightness. They’re not all exactly the same, but it turns out it’s even better than that. By studying them we can tell which ones are the 100 watt bulbs, and which are the 50 watts, and which are the 25 watt light bulbs. We can actually measure other properties of the explosion. And that is extremely valuable. So, they’re very bright, they have a narrow range of brightness, and when we see a supernova like that we can measure how bright it appears and figure out how far away it is.

This is very good because we want to figure out what the universe has been doing over time. What we know is that the universe is expanding. People have measured that the nearby galaxies are moving away from us. More distant galaxies are moving away from us more rapidly. It’s as if the whole universe is expanding in all directions. And we think that this is the result of, or anyway, the aftermath of the Big Bang, this sort of beginning of cosmic expansion about 14 billion years ago. And what we’re trying to do is to figure out how it’s changed over time. Has the universe expanded at a constant rate? Has it been slowing down due to gravity, as everyone expected it to? Or, as once – I don’t want to spoil the suspense, but what we’ve found is that the universe is actually speeding up, which is very mysterious.

So. The supernovae are really important because we use them to measure distances. If you can figure out the relation between velocity and distance, that is the expansion and time, you can actually trace the history of cosmic expansion.

Question: How did you determine from studying supernovae that the universe’s expansion is accelerating?

Robert Kirshner: Right. So, in the 1990’s, everybody thought that we would be able to use the supernovae to measure the slowing down of the universe because there had to be enough matter in there for the universe to be just bound and slowing down. Everybody kept telling us that’s what we would find.

So, we started to make the measurements and it was kind of interesting because the other group, the LBL group, published their results and said, “The universe is slowing down. We measured it with the supernovae and it seems to be slowing down.” So, that seemed good, except when we did our analysis of our data, we found that it didn’t seem to be slowing down. So, this was very awkward. We didn’t know what to do. We thought maybe we had made a mistake, or something. I don’t know.

But as time went on, going from 1997 into 1998, both teams started to talk about their results and both of us saw that in our data, things were not slowing down. It’s as if the universe had been speeding up over time. So it was slowing down at the beginning, I suppose, but then there’s been a switch from slowing down to speeding up. A kind of acceleration in the expansion of the universe.

Well, this was a big surprise to us. But it’s something that people had been talking about and had been thinking about for actually quite a long time. It turns out, Einstein, when he first applied General Relativity to the universes thought that the universe was equal to our Milky Way Galaxy because people didn’t know that these other galaxies were out there. So, when it came time to construct a model, a theory for how the universe would expand over time, he thought, static would be a good kind of – no expansion, that would be good. And he knew that if he had gravity that things would clump up, so he put into his equations another term to make things kind of a static universe. That was this so-called cosmological term, or cosmological constant. And there was no real reason that he did it, except it made it kind of easy to think about the universe as having a kind of finite volume and it avoided some technical problems and it made a static universe.

But what astronomers found in the next decade was this business of the galaxies rushing away from us. The expansion of the universe. So, at that point, say in the 1930’s, Einstein said, well forget the cosmological constant. If it doesn’t serve its purpose, then let’s not talk about it. But it does have this quality of making things speed up. The modern way to talk about it is as if it’s an energy associated with empty space. So you know, in quantum mechanics, there are a lot of weird things. And one of the weird things is that, if you look on smaller, and smaller and smaller scales, the uncertainty in the energy gets bigger and bigger. And that means, you don’t know when you’re looking at a little tiny piece of space whether it might have in it a particle and it’s anti-particle that get created out of nothing and then annihilate. On average, the value is zero, but the fluctuations are big. That’s what the quantum mechanics tells you. And the idea is, that there could be, the vacuum, is not just emptiness with no properties, but it could have, be sort of interesting properties. And that’s the modern theory of electro-magnetism. It’s like that with particles and there are anti-particles being created and destroyed all the time. And that picture fits the data, which have been exquisitely measured in laboratories on the earth, much better than a picture of a vacuum with no properties.

So this would be something similar for gravity. Now, nobody knows how to do this. Nobody knows how to make a proper quantum theory for gravity, so making the right prediction about what the properties of empty space ought to be is a little tricky. If you do it more or less the way people have for other forces in nature, it turns out you get a really bad answer because the energy associated with the vacuum gets big as the length that you’re looking at gets small. And it turns out that for gravity, the appropriate length is so small, and the energy that is associated with that is so big, that people have known since the first time they wrote this down that the universe isn’t like that.

So, what this says is, the cosmological constant is very embarrassing to theoretical physicists. The number they got was, first of all Einstein had said it was a bad thing, then when you did this calculation, the number they got was way too big, like 10 to the 60th, instead of .7, which is really bad. So, for a while, people just didn’t want to talk about it. They didn’t want to think about it and they thought, well, if it’s not infinite, it must be zero. That was one way to deal with it. And for an observational astronomer, like me, the idea was not to worry about cosmological constant because we thought we were going to measure slowing down.

Anyway, we measured cosmic speeding up, the natural way to get that is to have energy associated with the vacuum that is like the cosmological constant that can make the universe speed up over time. And that’s the idea that people have used as kind of a beginning to think about how to understand these observations that we really see a universe that’s expanding faster now than in the past. So, the idea is, there is something. We can call it the dark energy that resembles Einstein’s cosmological constant, more or less, that is really propelling the universe. That is making it expand. The trouble is, the way we talk about it, but we don’t understand it very well, is we don’t know – so if you do a calculation of what is the value of that number, you get a very wrong answer. So that’s telling you something’s not right. And there are plenty of other possibilities; it doesn’t have to be constant. There could be something that changes with time. You know, the dark energy could be any of a very large number of things that people have thought of. So there’s this kind of vast, weedy garden of ideas that people have thought of. Well, what about this, what about that, what about the other thing. And as an observer, what you want to do is do the best you can to put constraints on this and kind of weed out the garden. Show that some of these ideas are wrong and people shouldn’t waste their time pursuing them.

So the idea is with the observation – from the observational side was to kind of weed out this garden and find out which ideas are right, or anyway, could be right and show which ones are wrong so that people don’t waste their time working on those. So, that’s where we’re getting with this. We’ve seen the effect; the universe has been speeding up in the last five billion years or so. We’ve also looked far enough back into the past to see that the universe was slowing down before that, so that’s good. That’s a prediction of this kind of gravity and dark energy picture is that if you look far enough into the past, gravity should have been winning, that density should have been higher. So things sort of fit together.

But there was this very bad feeling that everybody has that we don’t understand in any basic way what this dark energy is. What’s more, there’s no physics experiments you can do. There’s no laboratory experiment that you can do, or that people have thought of anyway, that really tells you whether this idea is right – what the properties of the dark energy really are. So, it’s a very interesting thing where in astronomy, we see the effects, we talk about it as if it’s real, and we can try to pin down what the properties of the dark energy are. Is it like the cosmological constant? Does it change with time? That’s a good question you can try to work on that. And we are working on that. But there’s no laboratory experiment. It’s kind of a funny sort of thing where only the astronomy is telling us about this very big phenomenon that turns out to be very important. It’s two-thirds, or maybe three-quarters of the universe is in this form of the dark energy. Now that’s really weird and we discovered three-quarters of the universe in the last 10 years. We don’t know what it is. And there’s no laboratory experiment that helps us kind of sort this out.

Question: Based on our current picture of the universe, what do you predict its long-term fate will be?

Robert Kirshner: Right. Well, we’re very good at observing the past and telescopes really let you see light that was emitted 10 billion years ago. We do not see so clearly into the future, so if you make a prediction about what the universe is going to be like a billion years from now, it’s a little harder to test it.

Nevertheless, if the dark energy is really just the cosmological constant and there are no surprises, or another way to say it is, if the latest picture that we have in the last 10 years is absolutely perfect, which you know, a certain kind of modesty if probably a better policy. But never mind that. If we’ve got the whole story and we know exactly what the dark energy is, then you can predict what the expansion is going to be in the future. And it will be literally exponential expansion. That means the rate of expansion will depend on how big it is. It will go faster, and faster, and faster, and it’s like compound interest, or any of those things that kind of get out of hand. The expansion of the universe will get out of hand. And what that means, for astronomy is that distant galaxies, which are moving away from us at a fairly large fraction of the speed of light, will be moving away faster and faster, their light will get red shifted. That means that the light gets stretched out to longer wave lengths and they get dimmer and eventually we just won’t be able to see them.

So what will happen is that as time goes by, the distant galaxies will kind of go beyond the horizon for us. We won’t be able to see them. Another way to say this is the piece of the universe that we can see will kind of shrink in and we will have fewer and fewer things in it until finally – well finally – until, if you follow this idea far enough, it will just be us and the Andromeda Galaxy and we’ll be whirling around and eventually we’d collide with one another and there’d just be one galaxy, and that would be the whole universe.

And what’s so funny about it is that that is the picture that Einstein started with. That the Milky Way Galaxy was the whole universe. We will have come full circle. Also, it will be impossible at that date in the long distant future, 50 or 100 billion years in the future, it will be impossible for astronomers to sort this out because there won’t be any galaxies for them to observe. So, that’s why we have to do this now. That’s why increasing astronomy budgets in the coming decade is really an important thing because in 100 billion years, it will be impossible in principle to make these measurements.

Question: How much longer will the universe exist?

Robert Kirshner: Yeah. The time scale – the age of the universe now, we think is about 14 billion years. So, the universe has been expanding from this hot dense state in all directions, the Big Bang. And it’s been elaborated over time. Gravity has made things clump together so galaxies have formed, stars have formed, stars go through their lifecycle and they emit as they blow up as supernovae and put out heavy elements that are used by the next generation of stars and then they put planets. You have in your bones calcium atoms that were manufactured in supernovae, iron that’s in our blood, air that you’re breathing. Those nuclei, those actual atoms came from stars that blew up before the sun formed. So, you’re really part of this whole story, part of the universe. So that’s the story that we know has been going on in the past.

The piece in the future is much, much harder for us to say with confidence because after all, we always – there’s always that big voice that – astronomers know, the universe began in a powerful explosion 14 billion years ago. And that’s the voice in the planetarium and that’s the voice of authority. And it’s the voice of conventional wisdom. And we always talk like that. The trouble is, its what changes is nothing we say. So, now we say, “And the universe is expanding faster and faster—" Well, okay. That is our current picture, that is what we know and we’re trying to tell you the right story, but it would only take some very tiny deviation from the cosmological constant as the dark energy to produce a completely different affect. So, if there’s some slight, teeny little difference which we haven’t been able to measure, it could mean that the universe will expand for a while and then collapse in the future, or expand faster than exponentially, in which case things will get completely ripped apart. All of these weird things are possible, but our ability to measure them is really quite limited.

So it probably is a little bit better for us to be more modest about what we do know and what we can predict and what we can be sure is really going to happen. But I think we’re at a very interesting state where the evidence has gotten better over time, that we really live in a universe that’s really accelerating. The implication is that there is this stuff, this dark energy really, a negative pressure of something or other, in the universe. But you know, something that is related to the nature of gravity that we really don’t understand. There’s a big piece of fundamental physics that is missing. And this, in a way is great because it’s when you know everything and when everything is understood and it all fits together, that’s sort of a sign of something that’s done. Move on to something else.

When things don’t fit together and don’t quite make sense and you know that there’s a problem here, that’s great because of course, it means there’s going to be progress on this subject. The trouble is we don’t know when. And we don’t know which idea – at the moment, we don’t know which of the many ideas is really going to be the one that helps us solve this problem.

It probably will seem like a nutty idea when it’s new, you know, when we first hear about it, but that doesn’t mean that all nutty ideas are right. It just means we don’t have to figure out which of these things is really right. And to do that, we want to do big surveys to measure more precisely how the universe has been expanding. We want to measure precisely how matter has clumped together under the force of gravity. We want to study this history that’s available to us using telescopes to really see what the constraints are on this weird thing, this dark energy that seems so important. But that we really have very little grip on to – we have very little grip to tell which ideas are right and which ideas are wrong. And really, that’s what science is all about, is to pick out from the speculation and the kind of imagination which things agree with nature and so far, the measurements that we’ve made are kind of crude and we’ve got to do better.

Question: What came before the Big Bang?

Robert Kirshner: Yeah, people often ask that. I give a lot of public talks and there’s always somebody who stands up at the end and says, “Okay,” they say, “I got you now. What happened before the Big Bang?” And of course the answer is it’s not like we have some complete special knowledge and that we’re forcing you to believe it. We’re trying to puzzle this out. There is a time over which we really do understand the physics pretty well. And it goes surprisingly far back. That is, if you take a particle accelerator on the surface of the earth, and you smash together protons and antiprotons under the boarder of Switzerland and France, you are creating conditions which are like the ones that existed in the universe very, very early in a very small fraction of a second after the Big Bang.

So, I’m not saying we know our way all the way back to the beginning, and there are some mysteries there about exactly what happens when the density gets very, very high and what happens when the distances are so small that we really can’t predict what is going to happen. So, there are limits to our current understand of physics, but they are way back toward the beginning and there’s nothing that says that we won’t eventually understand more and more of it.

So, it’s kind of an evasive answer, I know that, but the fact is this idea that the universe was very dense, very hot at this early time is something for which the evidence is very strong and by pushing our current understanding of physics farther and farther back, we’re going to understand more and more of it. So, it’s a real adventure and that piece of it we really do have a program to study. The smashing together of particles in laboratory experiments really is creating conditions that were something like what was present back then at the time of the Big Bang, and will help us understand the universe on the big scale. So, this is very interesting idea that by looking on the very smallest scale, the smallest, tiniest fraction of the millimeter that we can do, we learn about the universe on the very biggest scale. The scale of the galaxies and the hundreds of millions, even billions of light years.

So, the interesting thing, or one interesting thing is that our understanding of the physical world is all of the piece. The understanding, the things on the smallest scale is really how we try to explain what’s going on, on the biggest scale.

Question: What was your greatest “Eureka!” moment, and how did it feel?

Robert Kirshner: Well, I can tell you something about the discovery of the cosmic acceleration which was that, Adam Reese, who was the post doc – had been my student and he was out of Berkeley, and he was doing the data analysis. And he’d call me up on the phone and he’d say, you know, this is kind of funny because I’m getting negative mass, you know. As if the universe is not slowing down, but speeding up. And I said, “Well Adam, you know. There’s lots of ways to go wrong and in your heart,” I said, “you know it’s wrong.” So, my approach to it was prejudice, unwillingness to pay attention – I just assumed he had forgotten to divide by the square root of pi of something and he would find his mistake and this thing would go away. And when it didn’t go away, it meant that we were – first of all, we’d read the paper from the other team and they said the universe is slowing down because of the super, you know, measured by the supernovae, so we had to go against that. We also had to go against the idea that Einstein had said. The cosmological constant was a terrible idea, you know. You shouldn’t talk about it, or as my mother said, “Do you think you’re smarter than Einstein?” I said, “No, mom.” So, this was terrible. Because we had to be confident enough that we would stand up and say this outrageous thing that the universe was accelerating, which was very surprising.

And there was another team and they had data, and if we didn’t do it, maybe they would do it, so we worked very hard and got our publication out early and we thought that was the right thing to do. But one of the things you don’t want to be as an observational astronomer, you don’t want to be wrong. You don’t want to tell people, I’ve seen this thing and it turns out not to be true. Or, as I told Adam, I think that the punishment for being wrong should be as big as the reward for being right. So, you want to do a job that’s reliable and you want to report something that’s accurate. You want to make sure that the statistics justify what you say and that you haven’t forgotten something or made some error.

And so, we were – in those early months there in 1998, I would say, it was very upsetting that this picture that the data were telling us, that we were living in an accelerating universe, and honestly, I did not like it, and I thought it was going to – it had the risk of being a really embarrassing, embarrassing thing. So, it’s worked out okay that 10 years later, the data is much better, the results are quite solid, and there are many other lines of evidence that are converging on this same view. Nevertheless, there’s that moment where, you know, you’re kind of afraid of the new thing. And you don’t want to be wrong. And you don’t want to be embarrassed. Nevertheless, eventually the data gets good enough and usually it’s the kids on the team, you know, the graduate students and the post docs who say, oh come on. Here’s the data, look at the data. Never mind what you think. Look at the data.

So, really new things in science are a little upsetting and kind of hard to come to terms with and for this thing, I actually was resisting for a while. So, it’s turned out okay. In the end I’ve come to like it. This is the universe we live in, at least the universe we think we live in. One that has this remarkable thing, this dark energy which is most of the universe. That we live in a universe that’s accelerating and maybe going into this kind of empty, cold future.

Question: What made you decide to become a physicist?

Robert Kirshner: Well, when I was in high school, I grew up in suburban Boston out in Sudbury, and it was a good high school. I was good at math and I was good in science. I had a neighbor who had a telescope and he had a little trouble setting it up, and so he asked if I wanted to help him do that. And we figured out how to point the axle at the North Pole and how to do all those things. And it was kind of exciting to look through a telescope and see things. What I did not understand at that point was there was actually something to do. It wasn’t just about seeing them and just about kind of knowing or looking at a natural history book, that you really could understand what those objects were. That didn’t come to me for a while until I was in high school and I remember reading books; reading books that Fred Hoyle had written; reading books that George Gamoff had written. You know popular science books that tried to bring you up to speed on what scientists were thinking. And that really got me going. That really got me going.

So, when I got to college, I came here to Harvard, as a freshman, I took a freshman seminar that was about astronomy and I’ve never really looked back. And I’d written a book. So, you know, maybe some kid will read it and would have made a perfectly good investment banker but goes astray and becomes an astronomer.